SPP1 and cancer: Under the influence of metastatic cancer cells, bone matrix factors such as osteopontin (OPN), osteonectin (ON), periostin (POSTN), bone sialoprotein (BSP), dentin matrix acidic phosphoprotein 1 (DMP1), and syndecan 1 are released to promote bone marrow cell recruitment, tumor migration, and survival [22,26,28].